A baseband synchronous digital train is in the form of a succession of signal periods of equal duration as defined by a bit rate and during which the signal remains at a constant level selected from a plurality of discrete values. Successive periods are separated by transitions. A geometrical pattern known as an eye diagram is obtained by superposing different periods of the digital train. Frequency limitations of a channel used for transmission purposes have the effect of distorting the digital train, with distortion appearing first at the transitions, thereby closing the eye diagram to a greater or lesser extent.
On reception, the periods are recognized by sampling their mid portions which are the least degraded by transmission, since they are furthest from the transitions. The mid portions of the periods correspond to the middle of the eye diagram.
However, in order to be able to sample the middles of the signal periods so as to facilitate recognition, it is necessary for the bit rate of the digital train to be recovered from the transitions in the train, either by mere filtering when the transmission code makes this possible, or else by nonlinear processing and filtering when the bit rate signal is accessible in the digital train only via its subharmonics.
The kind of nonlinear processing required, for example mere rectification when an HDB3 code is used, is not described in detail since the need for such processing and the manner in which it is defined are both functions of the code used, and are known to the person skilled in the art.
The filtering may be performed by means of a passive filter or by means of a phase-locked loop oscillator, and it is followed by or it includes means for adjusting the phase of the recovered bit rate signal so as to center the period sampling instants on the middle of the eye diagram, thereby providing the best possible immunity from transmission distortion.
Jitter (i.e. interfering phase modulation which displaces the transitions in the digital train from their normal positions) has unwanted effects on the bit rate signal and consequently reduces the accuracy of the instants at which the periods of the digital train are sampled, thereby increasing detection sensitivity to transmission distortion.
One known way of limiting this drawback is to attenuate the effect of digital train jitter on the recovered bit rate signal by using crystal filters, surface or volume wave filters, or phase-locked loop crystal oscillators, all of which are relatively expensive.
Preferred implementations of the present invention attenuate the sensitivity of the recovered bit rate to jitter in the originating digital train in a manner which is both simple and cheap.